Preferred embodiments of the present invention may be used to locate the standard transmitters used in a variety of communications networks. A "standard" transmitter is one which is generally used in the network and has not been specially designed or modified to make it easier to locate. For example, standard transmitters of the type mentioned above share two common characteristics.
First, they are intended to be mass marketed and thus inexpensive. To this end, they employ low cost transmitter and receiver circuitry. Such mass marketable transmitters are distinguished from more expensive transmitters designed specifically for use with a locating system. An example of the latter is disclosed in U.S. Pat. No. 5,218,618, Jun. 8, 1993, titled "Cellular Telephone Service Using Spread Spectrum Transmission," which discloses a system employing a special type of spread spectrum transmitter to be deployed in vehicles.
Second, each of the transmitters mentioned above employs a messaging channel that carries a bursty data transmission (as distinguished from a voice transmission or a circuit switch data transmission). The expression "messaging channel," as used herein, refers to one or more radio channels carrying two way communications between a first and a second radio transmitter (where either the first or the second radio transmitter is the transmitter to be located), wherein the communications include (1) commands and responses between the first and the second transmitters; and (2) information identifying at least one of the first and second transmitters. For example, a cellular telephone system's control channels are examples of "messaging channels." A CDPD system is a messaging system overlaid on a cellular system. Thus, the portion(s) of a cellular system's spectrum normally assigned to voice conversations may also be considered a messaging channel when carrying CDPD data packet transmissions. For further background on the present invention, the reader is referred to the above-cited U.S. patent application Ser. No. 08/059,248. In addition, reference may be made to U.S. patent application Ser. No. 08/105,506, filed Aug. 11, 1993, "Methods and Apparatus for Increasing the Efficiency of a Cellular Telephone Location System."